The technical field of this invention is dielectrometry and, in particular, devices for sensing changes in the dielectric properties of materials undergoing state transitions, such as resins and other materials undergoing curing during molding processes.
In the manufacture of parts molded from polymeric composites, it is advantageous to employ on-line monitoring devices to measure the progress of curing. For example, parts of optimal density and strength can require careful control of the heating rate, temperature gradients within the part, the timing and amount of applied pressure, and the cooling rate. In the past, control of these parameters has been conducted according to fixed schedules and often determined by trial-and-error methods.
A.C. measurements of dielectric properties by sensors implanted within a curing polymer can provide useful data on curing and other material properties. In particular, U.S. Pat. No. 4,423,371 issued to Senturia et al. in December 1983, discloses A.C. measurements in the frequency range of about 1 Hz to about 10 Hz and can be reliable indicators of curing. See also, U.S. Pat. No. 4,399,100 issued to Zsolnay et al. in August 1983, and U.S. Pat. No. 4,496,697 issued to Zsolnay et al. in January 1985, for further disclosures of automated process control systems for curing polymeric materials. Additionally, see U.S. Pat. No. 4,777,431 issued to Day et al. on Oct. 11, 1988, for a disclosure of a sensor adapted for at least partial implantation into the polymeric material undergoing curing to provide on-line monitoring of material property changes.
Typically, prior approaches to on-line monitoring of materials undergoing state transitions have required the implantation of a sensor into the polymeric material. This can lead to problems when the shape of the part does not permit placement of a dielectric sensor in either a portion of the part which will be trimmed off during subsequent finishing operations or into a region of the part where it will not affect the strength and be allowed to remain in the finished article. Additionally, this approach precludes the reuse of sensors.
Moreover, prior art sensors have not been well-suited for use in high pressure molding operations, such as reactive injection molding and the like, where the curing of the part occurs at high pressure that can damage or destroy the sensor element. Likewise, prior art embedded sensor have been impractical when the material contained abrasive components or fillers which can damage or destroy the casing or electrode structure. There exists a need for more reliable, abrasion-resistant, high temperature and high pressure-tolerant sensors for measuring dielectric property changes during manufacturing processes, especially molding operations to permit on-line monitoring and testing of materials.